Protective bellows

Abstract: A protective bellows (5) for protecting the steering mechanism of a motor vehicle body is shown. The bellows has a fixing collar (12) for gripping the outside of the casing (18) of the steering mechanism, and a fixing collar (14) for gripping a mounting region (20) of the steering rod (16). In order to be fitted into position, however, the fixing collar (14) has to be stretched to pass over a boss (22) of diameter A. The bellows is produced by blow-moulding from EPDM-polypropylene or similar thermoplastic material. Because of the lesser resilience of this material, compared with previously used materials such as thermoplastic polyurethane, the initial diameter of the fixing collar (14) has to be selected in dependence on the relative values for the diameters of the boss (22) and the mounting region (20) in order to ensure that the pressure with which the fixing collar (14) grips the mounting region (20) provides adequate sealing not only initially but throughout the expected life of the bellows.

Description: The invention relates to a protective bellows made of low-elasticity thermoplastic material in combination with and mounted on a rod-like member, the bellows having an integral fixing collar which has been moved axially along the rod-like member from an end thereof and resiliently fitted onto and around a circumferentially extending mounting surface of predetermined diameter on the rod-like member, the rod-like member also having a circumferentially extending intermediate surface which is axially positioned between the mounting surface and the end of the rod-like member and which has a diameter greater than the predetermined diameter of the mounting surface, so that the fixing collar has to be initially stretched to pass over the intermediate surface in order to be mounted onto the mounting surface. Such an arrangement is shown in EP-A-0 506 125. The bellows or boot in this arrangement has an internal engaging ring for engagement with an internal groove on a disc brake assembly and an external engaging ring for engagement with an external groove around the disc brake assembly. Compressed air and a special jig are used to semi-automate the process of mounting the internal and external engaging rings in position. Clearly, this is a complex operation involving the use of an expensive jig. It may be justified where special considerations apply in relation to a disc brake but is not suitable for more general fitting of a bellows to a rod-like member. The arrangement as first set forth above is also known from DE-U-8 807 327. Here, the fixing collar of a bellows is provided with an inwardly directed annular shoulder which is located in a groove around the casing of a steering mechanism. With normal types of elastic material, such as rubber, used for bellows, such an arrangement is satisfactory. However, it may be desirable for reasons of cost and otherwise to mould such bellows from thermoplastic material which may not resile satisfactorily onto the mounting surface if it is over-stretched to pass over the intermediate surface. The invention aims to deal with this problem. In accordance with the invention, therefore, the known bellows is characterised in that the initial diameter of the fixing collar is so selected in relation to the diameter of the intermediate surface and in relation to the elasticity of the thermoplastic material that the pressure which the resiled fixing collar exerts on the mounting surface exceeds a predetermined minimum value through the expected life of the bellows. Protective bellows embodying the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings in which: Figure 1 illustrates one of the bellows in cross-section and shows it being fitted into position on the steering mechanism of a vehicle; Figure 2 corresponds to Figure 1 but shows a later stage in the fitting process; Figure 3 corresponds to Figures 1 and 2 but shows the completion of the fitting process; and Figures 4 and 5 are graphs for explaining the operation of the bellows. The protective bellows to be described in more detail below is for protecting the steering mechanism in a motor vehicle body. The bellows is shown diagrammatically at 5 in Figure 1 and comprises a plurality of bellows turns 6,8 and 10 (but there may be more or less than this number of turns in practice), together with fixing collars 12 and 14 at each of its ends. The bellows is for protecting a steering rod 16 where it emerges from a casing 18 containing the manually, mechanically or hydraulically operated steering mechanism. The mechanism causes the steering rod 16 to move in an axial direction for controlling the steering angle of one of the front wheels of the motor vehicle. A similar steering rod projects from the opposite end of the casing 18 and is for controlling the steering angle of the other front wheel (and is protected by a bellows similar to the bellows 5). The bellows 5 is to be fitted onto the casing 18 and the rod 16 so that the collar 12 surrounds and is secured to the outside of the casing 18 and the collar 14 surrounds and is secured to a fixing region of the rod indicated at 20. As illustrated, though, the rod includes a boss 22 whose diameter A is greater than the diameter "B" of the fixing region 20 of the rod. As shown in Figures 2 and 3, therefore, the fitting process involves stretching the fixing collar 14 to enable it to pass over the boss 22 until it becomes positioned over the fixing region 20 of the rod 16 as shown in Figure 3. At the same time, the fixing collar 12 contacts the circumferential surface of the casing 18. The collars 12 and 14 are sized to hold themselves resiliently in position. It is advantageous to produce the bellows by blow-moulding from EPDM-polypropylene material or material of similar type, because such material has a lower cost than materials previously used such as thermoplastic polyurethane. However, polyurethane has greater elasticity than EPDM-polypropylene. If EPDM-polypropylene is used for the bellows 5, special dimensioning of the bellows is necessary, as will now be explained, in order to enable the material to cope with the stretching which is necessary to enable the fixing collar 14 to pass over the boss 22 (Fig. 2). Figure 4 plots the pressure applied by the fixing collar 14 to the region 20 of the shaft 16 as it varies during the life of the bellows - the applied pressure falling off during this time because of ageing effects in the material. The pressure value P3 is the minimum pressure necessary to ensure proper sealing of the bellows on the rod 16. As indicated in Figure 4, if the initial value of the pressure applied by the fixing collar 14 to the region 20 of the rod 16 is P2, then there will be adequate sealing over the first part of the life of the bellows. However, Figure 4 shows that, at a time t 1 , the pressure has fallen below the limit value P3, and adequate sealing is no longer available. However, if the initial value of the pressure is P1, Figure 4 shows that the pressure will be maintained above the minimum level P3 at least until a time T which is the maximum life of the bellows. The relationship between initial pressure value and final pressure value is determined by the material (EPDM-polypropylene) and the production technique used (blow-moulding). Therefore, if the bellows is produced by blow-moulding from EPDM-polypropylene, it is necessary to ensure that the initial pressure applied by the fixing collar 14 to the rod region 20 has a value of P1 at least - and the pressure will then be maintained above the minimum level for the life of the bellows. Figure 5 shows graphs plotting the stretching and resiling of the bellows material (vertical axis) against time (horizontal axis). The point B on the vertical axis represents the value of the diameter B (see Figures 1 to 3) of the rod region 20. Point A on the vertical axis represents the value of diameter A (Figs. 1 to 3), being the diameter of the boss 22. The curve X relates to a bellows for which the initial diameter of the collar 14 is "X". The diameter X is relatively small, and the effect of this is that the material becomes so stretched in passing over the boss 22 that, as shown by curve X, the diameter never resiles to the value B (the diameter of the rod region 20). Therefore, the bellows provides unsatisfactory sealing - even at the beginning of the life of the bellows. Curve Y corresponds to Curve X but relates to a bellows for which the diameter of the fixing collar 14 initially is Y. Curve Y shows how the material is stretched to pass over the diameter A of the boss 22 and then resiles back to the diameter B of the region 20. In this case, however, the pressure which the collar 14 applies to the region 20 of the rod 16 has the value P2. As shown by Figure 4, this pressure is unsatisfactory because it will fall below the minimum value P3 during the life of the bellows. Finally, curve Z relates to a bellows for which the initial diameter of the fixing collar 14 is Z. Again, the curve shows how the fixing collar is stretched to pass over the boss 22 with diameter A and then resiles to diameter B, the diameter of the mounting region 22 of the shaft. In this case, the pressure applied to the mounting region 20 is P1. As shown in Figure 4, this value will provide proper sealing throughout the life of the bellows. The time period t in Figure 5 shows the period during which the stretched collar resiles to grip the outer region 20. This time period needs to very short in order to ensure satisfactory fitting. It has been found that when the ratio of diameter A to diameter B lies between 1.4 and 1.8, the desired initial gripping pressure P1 is achieved when diameter Z satisfies the following relationship: 1.2 < (A - Z)/Z < 2

Claims: A protective bellows (5) made of low-elasticity thermoplastic material in combination with and mounted on a rod-like member (16), the bellows having an integral fixing collar (14) which has been moved axially along the rod-like member (16) from an end thereof and resiliently fitted onto and around a circumferentially extending mounting surface (20) of predetermined diameter (B) on the rod-like member, the rod-like member also having a circumferentially extending intermediate surface (22) which is axially positioned between the mounting surface (20) and the end of the rod-like member (16) and which has a diameter (A) greater than the predetermined diameter (B) of the mounting surface (20), so that the fixing collar (14) has to be initially stretched to pass over the intermediate surface (22) in order to be mounted onto the mounting surface (20), characterised in that the initial diameter of the fixing collar (14) is so selected in relation to the diameter (A) of the intermediate surface (22) and in relation to the elasticity of the thermoplastic material that the pressure which the resiled fixing collar (14) exerts on the mounting surface (20) exceeds a predetermined minimum value through the expected life of the bellows. A bellows according to claim 1, characterised in that the thermoplastic material is EPDM-polypropylene. A bellows according to claim 1 or 2, characterised in that the ratio (A/B) of the diameter of the intermediate surface (22) to the diameter of the mounting surface (20) lies between 1.4 and 1.8, and in that the initial diameter (Z) of the fixing collar (14) satisfies the following relationship, that is to say: 1.2 < (A - Z)/Z < 2 A bellows according to any preceding claim, characterised in that the rod-like member is a steering mechanism rod in a motor vehicle, in that the mounting surface (20) is the surface of the steering mechanism rod, and in that the intermediate surface is the surface of a boss (22) on the steering mechanism rod. A bellows according to claim 4, characterised by a fixing collar (12) at the other end of the bellows which is secured around a casing (18) for the steering mechanism from which protrudes the steering mechanism rod (16).

Inventors: BOYEN REMI, HOUIS JACQUES, BOYEN, REMI, HOUIS, JACQUES
Applicants: DRAFTEX IND LTD, DRAFTEX INDUSTRIES LIMITED
Application year: 1994
Main classification: F16J3
IPC codes: B29D23, F16J3, B29K23, F16L35